Bactericide for use in water treatment, method for water treatment and apparatus for water treatment

ABSTRACT

A water-treating microbicide, containing an inorganic acid and a corrosion inhibitor, and further containing a carboxylic acid having 8 or less carbon atoms or any of alkali metal salts thereof. The present invention can provide a water-treating microbicide, water treatment method and water treatment apparatus exhibiting a high sterilization effect in a membrane separation device for seawater desalination, etc.

TECHNICAL FIELD

[0001] The present invention relates to a water-treating microbicide, awater treatment method and a water treatment apparatus.

BACKGROUND ART

[0002] Membrane separation techniques are used in wide areas such asdesalination of seawater and brackish water, production of medical andindustrial pure water and ultrapure water, industrial wastewatertreatment and food industry. In such membrane separation, thecontamination of the separation device caused by microbes impairs thequality of the obtained permeating water, and furthermore promotes thegrowth of microbes on the membrane surface and the deposition ofmicrobes and their metabolites on the membrane surface, to lower thepermeability and separability of the membrane. To avoid these seriousproblems, various methods for sterilizing the membrane separation deviceare proposed, and generally, a microbicide is constantly orintermittently added to the feed liquid. As the microbicide, mostgenerally a chlorine-based microbicide advantageous in view of price andoperation is added to achieve a concentration of 0.1 to 50 ppm.Furthermore, an effective sterilization method, in which less expensivesulfuric acid is added to lower the pH of the liquid fed to the membraneseparation device to 4 or less, is also developed (EP1031372A). As thepiping of the membrane separation device, usually a corrosion resistantmetal such as stainless steel is used, but if the addition of sulfuricacid or the like raises the acidity, since the metal goes into thecorrosion region of the Pourbaix diagram, the piping is liable to becorroded. In a state where the acidity is low, there are such problemsthat the sterilization frequency must be increased and that longersterilization time is necessary for enhancing the sterilization effect.

DISCLOSURE OF THE INVENTION

[0003] The object of this invention is to overcome the above-mentioneddisadvantages of the prior art, by providing a water-treatingmicrobicide and a water treatment method having a high sterilizationeffect.

[0004] To solve the above problems, the present invention has thefollowing constitutions.

[0005] (1) A water-treating microbicide, comprising an inorganic acid, acorrosion inhibitor and a carboxylic acid having 8 or less carbon atomsor any of alkali metal salts thereof.

[0006] (2) A water treatment method, comprising the step of adding aninorganic acid, a corrosion inhibitor and a carboxylic acid having 8 orless carbon atoms or any of alkali metal salts thereof to a liquidundergoing treatment in any steps before a membrane separation step in awater treatment process using a separation membrane.

[0007] (3) A water treatment method, comprising the steps of adding aninorganic acid to a liquid undergoing treatment, to keep the pH at 4 orless intermittently, and adding a corrosion inhibitor to the liquidundergoing treatment, in any steps before a membrane separation step ina water treatment process using a separation membrane.

[0008] (4) A water treatment apparatus having a membrane separationdevice, comprising a means for adding an aqueous solution containing aninorganic acid and a corrosion inhibitor to a liquid undergoingtreatment to be fed to said membrane separation device.

[0009] (5) A water treatment apparatus having a membrane separationdevice, comprising a means for feeding an aqueous solution containing anacid and a means for feeding an aqueous solution containing a corrosioninhibitor, respectively to the liquid fed to said membrane separationdevice.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIG. 1 is a schematic drawing showing the water treatmentapparatus of this invention.

[0011] Meanings of symbols

[0012] 1 water intake device

[0013] 2 liquid feed pump

[0014] 3 pre-treatment device

[0015] 4 intermediate layer and safety filter

[0016] 5 booster pump

[0017] 6 membrane separation device

[0018] 7 liquid feed pump

[0019] 8 post-treatment device

THE BEST MODES FOR CARRYING OUT THE INVENTION

[0020] In the present invention, water treatment refers to a processsuch as the desalting, separation or desalination of seawater orbrackish water, production of industrial pure water or ultrapure water,industrial wastewater treatment, separation or concentration in the foodindustry, or recovery of valuable materials from wastewater.

[0021] In this invention, a membrane separation device refers to adevice, in which a liquid undergoing treatment is supplied to a membranemodule under pressurization for separation into a permeating liquid anda concentrate for the purpose of fresh water generation, concentrationor separation, etc. Examples of the membrane module include a reverseosmosis membrane module, ultrafiltration membrane module,microfiltration membrane module, etc. The membrane separation devicescan be classified into a reverse osmosis membrane device,ultrafiltration membrane device and microfiltration membrane device,mainly in reference to the membrane module used.

[0022] A reverse osmosis membrane device preferably used in thisinvention is described below as an example. A reverse osmosis membranedevice usually consists of a reverse osmosis membrane element, pressurevessel, booster pump, etc. The liquid undergoing treatment to be fed tothe reverse osmosis membrane device usually contains chemicals such as amicrobicide, coagulant, reducing agent and pH regulator, and it ispretreated by means of coagulation, settling, sand filtration, polishingfiltration, active carbon treatment, microfiltration, ultrafiltration,safety filter permeation, etc., before being fed into the device. Forexample, in the case of seawater desalting, seawater is taken in andseparated from particles, etc. in a settling basin, and a microbicidesuch as chlorine is added to the settling basin for sterilization. Insuccession, a coagulant such as iron chloride or polyaluminum chlorideis added, and sand filtration is carried out. The filtrate is stored ina storage tank and adjusted in pH using sulfuric acid, etc., to be fed.While it is fed, a reducing agent such as sodium hydrogensulfite isadded to reduce and remove the microbicide, and the residue is permeatedsafety filter. Then the filtrate is raised in pressure by ahigh-pressure pump and fed into a reverse osmosis membrane module.However, these pre-treatments are selected as required, depending on theliquid to be treated, application, etc.

[0023] The reverse osmosis membrane refers to a semi-permeable membranethat allows a component in the liquid such as a solvent to permeate butdoes not allow the other components to permeate. The materials generallyused as the reverse osmosis membrane include high molecular materialssuch as cellulose acetate polymers, polyamides, polyesters, polyimidesand vinyl polymers. As for the structure of the membrane, there are, forexample, an asymmetric membrane having a dense layer at least on oneside of it and having fine pores gradually increasing in diameter fromthe dense layer into the membrane or toward the other side, and acomposite membrane consisting of said asymmetric film and a very thinactive layer made of another material formed on said dense layer. As forthe form of the reverse osmosis membrane, there are hollow fibermembranes, flat membrane, etc. Usually it is preferred that the membranethickness of hollow fiber membranes or flat membrane is 10 μm to 1 mm,and that the outer diameter of the hollow fiber membranes is 50 μm to 4mm. As a flat membrane, an asymmetric membrane is preferred, and as acomposite membrane, it is preferred that a substrate such as a wovenfabric, knitted fabric or non-woven fabric, etc. is used as a support.However, the method of the present invention can be used irrespective ofthe material, structure and form of the reverse osmosis membrane, and iseffective in every case.

[0024] Typical examples of the reverse osmosis membrane include acellulose acetate- or polyamide-based asymmetric membrane, and acomposite membrane having a polyamide- or polyurea-based active layer.Among them, a cellulose acetate-based asymmetric membrane and apolyamide-based composite membrane are especially effective for themethod of this invention, and an aromatic polyamide-based compositemembrane is further more effective.

[0025] A reverse osmosis membrane module is a product formed foractually using the above-mentioned reverse osmosis membrane. In the casewhere the reverse osmosis membrane is formed as a flat membrane, it canbe installed in a spiral, tubular or plate-and-frame module, and in thecase of hollow fiber membranes, they are bundled and installed in amodule. The present invention can be applied irrespective of theseconstitutions of reverse osmosis membranes.

[0026] The operation pressure of a reverse osmosis membrane device isusually in a range of 0.1 MPa to 15 MPa, and can be selected asrequired, depending on the liquid to be treated, operation method, etc.In the case where a solution with a low osmotic pressure such asbrackish water is going to be treated, the device is operated at arelatively low pressure, and in the case where seawater or industrialwastewater is going to be treated, it is operated at a relatively highpressure.

[0027] It is preferred that the operation temperature of the reverseosmosis membrane device is in a range of 0° C. to 100° C. If thetemperature is lower than 0° C., the liquid undergoing treatment may befrozen, and if higher than 100° C., the liquid undergoing treatment mayevaporate.

[0028] The recovery of the liquid undergoing treatment in the reverseosmosis membrane device can be usually selected in a range of 5 to 98%.However, the pre-treatment methods and operation pressure must be takeninto account in reference to the properties, concentrations and osmoticpressures of the liquid undergoing treatment and the concentrate, whenthe recovery is set. For example, in the case of seawater desalination,a recovery of 10 to 40% is usually set, and in the case of highlyefficient device, a recovery of 40 to 70% is set. In the case ofbrackish water desalination or production of ultrapure water, operationcan be made usually at a high recovery of 70% or more, as required at 90to 95%. The recovery refers to a value obtained by dividing the amountof the liquid permeating the reverse osmosis membrane by the amount ofthe liquid undergoing treatment, and multiplying the quotient by 100.

[0029] A reverse osmosis membrane device mainly consists of ahigh-pressure pump and a reverse osmosis membrane module. As thehigh-pressure pump, an optimum pump can be selected in response to theoperation pressure of the device.

[0030] As the reverse osmosis module, one module can be used, but it ispreferred to use plural modules disposed in series or parallel to theliquid undergoing treatment. In the case where they are disposed inseries, a booster pump can be installed between the reverse osmosismembrane modules. In the case of seawater desalination, in view ofequipment cost, especially two modules disposed in series can bepreferably used. In this case, it is preferred to install a booster pumpbetween the reverse osmosis membrane modules disposed in series, forraising the pressure of the liquid undergoing treatment to 1.0˜5.0 MPa,when feeding it to the latter module. If the reverse osmosis membranemodules are disposed in series to the liquid undergoing treatment, theeffect of the present invention is large, since the time during whichthe liquid undergoing treatment is kept in contact with the membranemodules becomes long.

[0031] Furthermore, the reverse osmosis membrane modules can also bedisposed in series to the permeating liquid. This is a preferable methodin the case where the quality of the permeating liquid is insufficientfor use or in the case where it is intended to recover the solute in thepermeating liquid. In the case where the reverse osmosis membranemodules are disposed in series to the permeating liquid, it is preferredto install a pump between the reverse osmosis membrane modules, forre-pressurizing the permeating liquid, or for applying a sufficientpressure in the former step for using the remaining pressure in thelatter step for membrane separation. Furthermore in the case where thereverse osmosis membrane modules are disposed in series to thepermeating liquid, it is preferred to install an acid-adding devicebetween the reverse osmosis membrane modules for sterilizing the latterreverse osmosis membrane module.

[0032] In the reverse osmosis membrane device, the portion notpermeating the membranes out of the liquid undergoing treatment is takenout as a concentrate from the reverse osmosis membrane modules. Theconcentrate can be used or thrown away, or can also be furtherconcentrated by any other method. The concentrate can also be partiallyor wholly circulated into the liquid undergoing treatment. Thepermeating liquid that has permeated the membranes can be used, thrownaway or can also be partially or wholly circulated into the liquidundergoing treatment.

[0033] In general, the concentrate of the reverse osmosis membranedevice has pressure energy, and for reducing the operation cost, it ispreferred to recover the energy. The energy can be recovered by means ofan energy recovery device attached to any desired high-pressure pump,but it is preferred to recover the energy by a special turbine typeenergy recovery pump installed before or after a high-pressure pump orbetween modules.

[0034] It is preferred that the treatment capacity of the membraneseparation device used in this invention is 0.5 m³ to 1,000,000 m³ asthe amount of water treated per day.

[0035] Furthermore, in the membrane separation device used in thisinvention, it is preferred that the piping in the device has a structurewith few retaining portions.

[0036] In the water treatment method of this invention, an inorganicacid and a corrosion inhibitor are intermittently added to the liquidundergoing treatment to be fed into the water treatment apparatus. Theaddition of an inorganic acid is very important in view of giving thesterilization effect, and the effect is remarkable especially in themembrane filtration using seawater as the liquid undergoing treatment.The pH at which microbes are killed is peculiar to each microbe species.For example, in the case of Escherichia coli, the lower limit of pH forgrowth is 4.6, but the bacterium will be killed at pH 3.4 or less.Seawater contains many kinds of microbes, and they are killedrespectively at different pH values. However, usually if the liquidundergoing treatment is kept at pH 4.0 or less for a certain period oftime, 50 to 100% of microbes can be killed. It is preferred that the pHof the liquid undergoing treatment containing an inorganic acid and acorrosion inhibitor is 3.9 or less. More preferred is 3.7 or less, andespecially preferred is 3.4 or less. There is no particular limit forthe lower limit of pH, but in view of preventing the corrosion ofequipment, 1.5 or more is preferred, and especially 2.0 or more ispreferred.

[0037] Furthermore, it is preferred that the pH of the liquid undergoingtreatment is 3.0 or less, for presenting a high sterilization effectagainst microbes including aciduric microbes. If the pH is kept constantat 3.0 or less, a high sterilization effect against all the microbesincluding aciduric microbes can be shown, but the chemical cost formaking the feed liquid acidic becomes high while the effect on thecorrosion of piping equipment threatens to be large. So, it is preferredin view of efficient sterilization, that during ordinary intermittentsterilization, the pH of the liquid undergoing treatment is kept at ashigh as higher than 3.0 in a range of 3.0 to 4.0, and that against themicrobes still remaining to live without being killed, the liquidundergoing treatment is kept at 3.0 or less at a frequency of once per 2to 1,000 times of intermittent sterilization.

[0038] It is preferred to intermittently add an inorganic acid and acorrosion inhibitor to the liquid undergoing treatment to ensure thatthe plate count remaining rate in the concentrate is kept at 30% or lessafter completion of membrane separation, and that the plate countremaining rate is kept at 15% or less at a frequency of once per 2 to1,000 times of intermittent addition of an inorganic acid. If the platecount remaining rate is more than 30%, sterilization is insufficient.The plate count remaining rate (%) is obtained from the followingformula.

Plate count remaining rate (%)={(Plate count after adding the inorganicacid)/(Plate count before adding the inorganic acid)}×100

[0039] As the inorganic acid used in this invention, any of hydrochloricacid, sulfuric acid, nitric acid, phosphoric acid, etc. can be used, butin view of economic aspect, the use of sulfuric acid is preferred.

[0040] The corrosion inhibitor used in this invention is important forpreventing the corrosion of the water treatment apparatus and raisingthe sterilization effect. As the corrosion inhibitor used in thisinvention, a compound selected from polycarboxylic acids having at leastsix carboxylic acid groups in the molecule, ethylenediaminetetraaceticacid, nitrous acid and their alkali metal salts can be preferably used.As the polycarboxylic acid, at least one compound selected frompolyepoxysuccinic acids represented by the following general formula(where n is an integer of 3 or more, and X and Y denote, respectivelyindependently, hydrogen or alkali metal), polyacrylic acid, polymaleicacid, and maleic acid copolymers can be preferably used.

[0041] As the corrosion inhibitor, a compound selected frompolyepoxysuccinic acids, ethylenediaminetetraacetic acid, polyacrylicacid and their alkali metal salts is especially preferred. Since thesecompounds have atoms with high electro-negativity such as oxygen andnitrogen in the molecule, they are preferably excellent in adsorbabilityto the surface of a metal.

[0042] Among them, polyacrylic acid is most preferred since it has highfood safety and high corrosion-inhibiting effect. Polyacrylic acid isespecially preferred in the case where water treatment is intended forproduction of drinking water.

[0043] The optimum range of the weight average molecular weight ofpolyacrylic acid depends on water treatment conditions such as pH andtemperature. So, it is necessary to select polyacrylic acid having aweight average molecular weight suitable for the conditions. It ispreferred that the weight average molecular weight of polyacrylic acidis in a range of 500 to 10,000, and a more preferred range is 1,000 to8,000. If the weight average molecular weight is less than 500, it isdifficult to obtain a sufficient corrosion-inhibiting effect, and ifmore than 10,000, the storage stability of the microbicide is likely tobe low.

[0044] A polyepoxysuccinic acid or any of alkali metal salts thereof canbe synthesized, for example, according to the following method. Amaleate is epoxidized using hydrogen peroxide and also using sodiumtungstate as a catalyst, to make an epoxysuccinate. Then, theepoxysuccinate is polymerized with ring opening using calcium hydroxideas a catalyst in an alkali aqueous solution, to obtain apolyepoxysuccinate. As the maleic acid copolymer, a copolymer consistingof maleic acid and an olefin, a copolymer consisting of maleic acid andmethyl vinyl ether, etc. can be preferably used.

[0045] The acid and the corrosion inhibitor can be added separately tothe liquid undergoing treatment to be fed to the water treatmentapparatus, or a water-treating microbicide containing both mixedbeforehand can also prepared and added. The preparation of awater-treating microbicide in advance is preferred, since thesterilization treatment can be carried out efficiently.

[0046] It is preferred that the concentrations of the inorganic acid andthe corrosion inhibitor in the water-treating microbicide of thisinvention are in a range of 50 ppm (weight) to 50 wt % respectively. Ifthe concentration of each or either of the acid and the corrosioninhibitor is more than 50%, the storage stability of the microbicide islikely to decline. If the concentration of each or either of the acidand the corrosion inhibitor is less than 50 ppm, it is necessary toincrease the added amount of the water-treating microbicide, and thesterilization efficiency is likely to decline.

[0047] It is preferred that the water used in the water-treatingmicrobicide of this invention is pure water. If the water used containsimpurities, they may react with the acid or corrosion inhibitor, to forma precipitate, and the storage stability may decline.

[0048] Since the mixture consisting of an acid and a corrosion inhibitorcan happen to be poor in storage stability, it is preferred to furtheradd a storage stabilizer to the water-treating microbicide. As thestorage stabilizer, for decreasing the damage to the separationmembranes of the water treatment apparatus and for sustaining thesterilization effect, a carboxylic acid having 8 or less carbon atoms orany of alkali metal salts thereof can be preferably used. As thecarboxylic acid having 8 or less carbon atoms, preferred is at least oneselected from acetic acid, lactic acid, succinic acid, tartaric acid,citric acid and malic acid. If such a storage stabilizer is added, themicrobicide containing an acid and a corrosion inhibitor can be storedstably for a long period of time. The optimum range of the concentrationof the storage stabilizer in the water-treating microbicide depends onthe concentrations of the acid and the corrosion inhibitor in themicrobicide, but it is usually preferred that the concentration is in arange of 50 ppm (weight) to 50 wt %.

[0049] The water-treating microbicide of this invention can be used invarious water treatment processes, but it is preferred to use themicrobicide in a water treatment process using a separation membranegreatly affected by microbes.

[0050] The separation membranes that can be used in this inventioninclude reverse osmosis membranes, ultrafiltration membranes,microfiltration membranes, etc., but it is preferred that thewater-treating microbicide is used in a water treatment process usingreverse osmosis membranes for which a generally used oxidizing agentsuch as chlorine cannot be used.

[0051] The acid and the corrosion inhibitor can be added separately tothe liquid undergoing treatment to be fed to the water treatmentapparatus, or a water-treating microbicide containing both mixedbeforehand can also prepared and added. The preparation a water-treatingmicrobicide in advance is preferred, since the sterilization treatmentcan be carried out efficiently.

[0052] It is preferred that the water-treating microbicide is added at aconcentration in a range of 10 ppm (weight) to 10 wt % in the liquidundergoing treatment. If the added amount of the microbicide is smallerthan 10 ppm, it is necessary to raise the concentrations of the acid andthe corrosion inhibitor in the microbicide, for obtaining a highsterilization effect, and in this case, the storage stability of thewater-treating microbicide may decline. If the added amount of thewater-treating microbicide is larger than 10 wt %, a large load acts onthe device used for adding the water-treating microbicide, and theenergy consumption becomes large. This may be an economic disadvantage.

[0053] It is preferred that the water-treating microbicide is addedintermittently. It is preferred that the period of time during which themicrobicide is kept added each time is in a range of 0.5 to 2.5 hours,and that the addition frequency is once per day to per month. It ispreferred to adequately change the addition period of time and additionfrequency, monitoring the variation in the amount of water permeatingthe membrane, the variations in the plate count and contained organiccarbon of the concentrate, the rise of differential pressure, etc. Forsterilizing the membranes, the membranes can be immersed in an aqueoussolution containing an acid and a corrosion inhibitor while the watertreatment apparatus stops operation, but the method of adding thewater-treating microbicide to the liquid undergoing treatment duringmembrane separation is efficient and preferred.

[0054] In the water treatment method of this invention, the inorganicacid and the corrosion inhibitor can also be added separately to theliquid undergoing treatment. It is preferred that the amount of theinorganic acid added to the liquid undergoing treatment is 10 ppm(weight) or more in view of sterilization effect, and is 1 wt % or lessin view of economy and the corrosion prevention of equipment such aspiping.

[0055] The preferred amount of the microbicide added to the liquidundergoing treatment depends on the salt concentration of the liquidundergoing treatment, but it is preferred to control to ensure that thepH of the liquid undergoing treatment becomes 4 or less intermittently,and that the concentration of the corrosion inhibitor in the liquidundergoing treatment is kept in a range of 0.1 ppm to 1%. If the pH ofthe liquid undergoing treatment becomes higher than 4, the sterilizationeffect can happen to decline. Furthermore, if the concentration of thecorrosion inhibitor is lower than 0.1 ppm, the corrosion-inhibitingeffect may decline. On the contrary, if the concentration of thecorrosion inhibitor is higher than 1%, the corrosion- preventing effecttends to level off, and this may be an economical disadvantage.

[0056] If sulfuric acid is used as the inorganic acid, it is preferredto keep the added amount proportional to the salt concentration of theliquid undergoing treatment. For example, when 50 ppm of sulfuric acidwas added to a pressure-sterilized (120° C., 15 minutes) physiologicalsalt solution (salt concentration 0.9 wt %), the pH dropped to 3.2, butwhen seawater samples collected at three places and a commerciallyavailable artificial seawater sample (salt concentration about 3.5 wt %)were pressure-sterilized (120° C., 15 minutes) and used as liquidsundergoing treatment, the pHs of the liquids undergoing treatment werein a range of 5.0 to 5.8 even if 100 ppm of sulfuric acid was added.This is considered to be mainly the effect due to the M alkalinity ofseawater. To keep the pH of seawater at 4 or less, it is usuallypreferred to add 120 ppm (weight) or more of sulfuric acid. It ispreferred that the upper limit in the added amount of sulfuric acid is400 ppm or less in view of economy and the corrosion prevention ofequipment such as piping. More preferred is 300 ppm or less. When theconcentrations of sulfuric acid added to the above natural seawater andartificial seawater samples were 150 ppm and 200 ppm, the pH values ofthe liquids undergoing treatment were respectively 3.2 to 3.6 and 2.8 to2.9. That is, with the increase in the concentration of sulfuric acid,the pH variation of the liquid undergoing treatment decreases.

[0057] The optimum range of the concentration of the corrosion inhibitorin the liquid undergoing treatment depends on the liquid to be treatedand water treatment conditions, but a range of 0.1 ppm (weight) to 1 wt% is usually preferred. In view of economy and the convenience of watertreatment operation, a range of 1 to 500 ppm is more preferred. Forexample, in the case where wastewater with a pH of 1.0 and a saltconcentration of about 8% is treated at a temperature of 35° C., it ispreferred that the concentration of the corrosion inhibitor is in arange of 1 to 100 ppm in the liquid undergoing treatment.

[0058] In this invention, the inorganic acid and the corrosion inhibitorcan be added at any step before the liquid undergoing treatment is fedto the membrane separation device. For sterilization of the membraneseparation device, it is preferred to add them immediately before themembrane separation device. Furthermore, it is preferred to add theinorganic acid on the downstream side of adding the corrosion inhibitorto the liquid undergoing treatment, for inhibiting the corrosion ofpiping.

[0059] It is also a preferred method to add the corrosion inhibitorsimultaneously when the inorganic acid is added. In the case where thecorrosion inhibitor is expensive, it is preferred, in view of economy,to add it only when the pH of the liquid undergoing treatment is 3.0 orless.

[0060] It is preferred to intermittently add the inorganic acid and thecorrosion inhibitor. It is preferred that the period of time duringwhich the inorganic acid and the corrosion inhibitor are kept added eachtime is in a range of 0.5 to 2.5 hours, and that the addition frequencyis once per day to per month. It is preferred to adequately change theaddition period of time and addition frequency, monitoring the variationin the amount of water permeating the membranes, the variations in theplate count and contained organic carbon of the concentrate, the rise ofdifferential pressure, etc. For sterilizing the membranes, the membranescan be immersed in an aqueous solution containing the acid and thecorrosion inhibitor while the water treatment apparatus stops operation.

[0061] In the case where the inorganic acid and the corrosion inhibitorare added separately, the addition frequency of the inorganic acid canbe different from that of the corrosion inhibitor. For example, the acidcan be added for 0.5 to 2.5 hours every other day, and the corrosioninhibitor can be added for the same period of time but at a differentfrequency, say, once per week. Especially if the corrosion inhibitor isexpensive and excellent in the corrosion-inhibiting effect, it ispreferred to lower the addition frequency of the corrosion inhibitor inview of economy, for example, to combine the addition of the acid onlyand the addition of both the acid and the corrosion inhibitor.

[0062] The water treatment apparatus having a membrane separation deviceof the present invention consists of, for example, the following A to H.

[0063] A. Water intake device: This device takes in the liquidundergoing treatment as the raw water, and usually consists of an intakepump, chemical injection equipment, etc.

[0064] B. Pre-treatment devices communicating to the intake device:These devices pre-treat the liquid undergoing treatment to be fed to themembrane separation device, for removing the suspended matter,emulsified product, etc. in the liquid undergoing treatment, and injectsome chemicals. For example, the devices are disposed in the followingorder.

[0065] B-1 Coagulation filtration device

[0066] B-2 Polishing filter

[0067] An ultrafiltration device and a microfiltration device can alsobe used instead of B-1 and B-2.

[0068] B-3 Chemical injecting equipment for injecting a coagulant,microbicide, pH regulator, etc

[0069] C. Intermediate tank installed as required to communicate to thepre-treatment devices: Having such functions as water quantity controland water quality buffer action.

[0070] D. Filter communicating to the intermediate tank, if it isinstalled, or to the pre-treatment devices, if the intermediate tank isnot installed: Having a function of removing solid impurities of theliquid undergoing treatment to be fed to the membrane separation device.

[0071] E. Membrane separation device: Consisting of a high-pressure pumpand a membrane separation module.

[0072] Plural membrane separation devices can also be installed inparallel or in series. If they are installed in series, a pump can beinstalled between the membrane separation devices for raising thepressure of the liquid undergoing treatment to be fed to the lattermembrane separation device.

[0073] F. Post-treatment devices communicating to the permeating wateroutlet of the membrane separation device. For example, the followingdevices can be exemplified.

[0074] F-1 Degasifier: Having a function of removing carbonic acid

[0075] F-2 Calcium column

[0076] F-3 Chlorine-injecting device

[0077] G. Post-treatment devices communicating to the outlet on the rawwater side of the membrane separation device. For example, the followingdevices can be exemplified.

[0078] G-1 Buffer: For example, a neutralizer

[0079] G-2 Discharge equipment

[0080] H. Others

[0081] A wastewater treatment device, etc. can also be installed asrequired.

[0082] The water treatment apparatus of this invention can have a pumpinstalled at a desired place. Furthermore, it is preferred that one ormore means for adding the inorganic acid and the corrosion inhibitor ortheir aqueous solutions are installed in the intake device A or thepre-treatment devices B or before the pre-treatment devices B, or beforeor after the filter D. Especially it is preferred to install the meansbefore the membrane separation device, i.e., before or after the filterD.

[0083] To enhance the effect of this invention, it is preferred that thedevices used for adding the water-treating microbicide, inorganic acidand corrosion inhibitor can be automatically controlled, and arerespectively provided with a pump capable of adequately controlling theinjected amount. Furthermore, it is preferred to install the instrumentsfor measuring the pH values of the liquid undergoing treatment to be fedand the concentrate, the concentration of the corrosion inhibitor, etc.in the apparatus. Moreover, to control the intermittent addition ofwater-treating microbicide, etc., it is preferred that an instrumentcapable of measuring time is provided. It is more preferred that anautomatic controller allowing the automatic operation of the watertreatment apparatus as a whole is provided.

[0084] It is preferred that the components of the water treatmentapparatus of this invention such as piping and valves are made ofmaterials unlikely to be corroded at pH 4 or less. If the liquidundergoing treatment to be fed is kept at pH 4 or less, a highsterilization effect can be obtained, and the effect of removing thescale in the piping can also be obtained. To prevent the membranedeterioration caused by oxidizing agents of chlorine, etc., sodiumhydrogensulfite is added as the case may be, but, since thewater-treating microbicide of the present invention is used, the amountof sodium hydrogensulfite can be remarkably decreased.

[0085] The addition of a chlorine-based microbicide in a pre-treatmentstep is effective for sterilization and is generally used. In the caseof a treatment apparatus having a membrane separation device, achlorine-based microbicide is continuously or intermittently injected,for example, in any step of said devices A to D. This method can almostperfectly sterilize the liquid undergoing treatment to be fed unless anyresistant strain emerges. A chlorine-based microbicide can chemicallydeteriorate a reverse osmosis membrane, and to prevent it, a reducingagent such as sodium hydrogensulfite is generally added immediatelybefore the membrane separation device. However, in the liquid undergoingtreatment remaining after reducing and removing chlorine by a reducingagent, microbes can easily grow. In addition, the microbes are not avariety of microbes existing in the raw seawater before the addition ofthe microbicide, but a group of very sorted microbes that may includemany aciduric microbes exist there. This problem can be solved if theaddition of a chlorine-based microbicide in the pre-treatment step andthe injection of a reducing agent immediately before the membraneseparation device are carried out respectively intermittently. Thismethod is also effective for preventing the membrane deterioration. Itis preferred that the chlorine-based microbicide is injected once perday to per six months for about 30 minutes to 2 hours each time, inreference to the quality of raw seawater, i.e., the existence ofmicrobes. In adaptation to the timing of adding the chlorine-basedmicrobicide, and considering the movement of the water containing thechlorine-based microbicide, it is preferred to supply a reducing agentat a position between the pre-treatment devices and the membraneseparation device for inactivating the chlorine-based microbicide. Inaddition, in adaptation to the timing, it is desirable to add thewater-treating microbicide of this invention or to add the corrosioninhibitor and the acid separately to the aqueous solution to be fed tothe membrane separation device, for sterilizing the membrane separationdevice.

[0086] The intermittent chlorine-based microbicide injection method tothe pre-treatment step like this gives an effect of remarkablydecreasing the treatment cost such as microbicide cost compared with thecontinuous injection of microbicide. This can be achieved for the firsttime with the water treatment method of the present invention using thewater-treating microbicide or the acid and the corrosion inhibitor, andcould never be achieved by the conventional sterilization methods sincethe sterilization effect is insufficient.

[0087] The water treatment method and apparatus of this invention can besuitably used for the water treatment with a membrane separation device.Particularly it can be suitably used for water refining processes suchas desalination of seawater, desalination of brackish water, productionof industrial water, production of ultrapure water or pure water,production of medicinal pure water, clarification of raw tap water, andadvanced treatment of tap water. Furthermore also in the concentrationof food, also in the case where organic materials, etc. likely to bedecomposed by conventional oxidizing microbicides are separated orconcentrated, they can be concentrated or recovered without beingdecomposed. So, the effect of this invention is large. Moreover, in thecase of producing drinking water, this invention has an effect that thegeneration of the trihalomethane produced with chlorine sterilizationcan be prevented. Still furthermore, the water treatment method of thisinvention is especially suitable for the production of drinking water,since compounds with high food safety only can be used forsterilization.

EXAMPLES

[0088] This invention is described particularly in reference toexamples, but is not limited thereto or thereby.

[0089] First of all, the synthesis of the chemical solution used in theexamples is described below.

[0090] <Example of Synthesizing a Polyepoxysuccinate>

[0091] An epoxysuccinate was synthesized as described below according tothe method of Payne, et al. (J. Org. Chem., 24, 54 (1959)).

[0092] A 2-liter three-neck flask was charged with 280 g of maleicanhydride and 428 ml of ultrapure water for dissolution. To the aqueoussolution, 500 g of 48 wt % potassium hydroxide aqueous solution wasadded dropwise using a dropping funnel with cooling to keep thetemperature at room temperature. Then, 18.8 g of sodium tungstate wasadded, and subsequently 332 g of 35% hydrogen peroxide water was addeddropwise. The mixture was stirred for about 30 minutes, and 115 g of 48wt % potassium hydroxide aqueous solution was gradually added. In thiscase, the flask was quickly cooled to keep the reaction temperature at55 to 65° C. Then, the reaction mixture was kept at 65 to 60° C. for 30minutes, to obtain a potassium epoxysuccinate aqueous solution. Theaqueous solution was cooled to room temperature and concentrated to 300ml, and it was poured into 1 liter of acetone. The produced precipitatewas secured by filtration, for isolation as potassium epoxysuccinate.

[0093] Then, a 200 ml round bottom flask was charged with 10.4 g of thepotassium epoxysuccinate and 50 g of ultrapure water, and 48 wt %potassium hydroxide was added, to adjust the pH of the aqueous solutionto 10.3. Furthermore, 0.41 g of calcium hydroxide was added, andreaction was carried out at 80° C. for 6 hours. In succession, thereaction mixture was cooled to room temperature, and the insolublematter was secured by filtration. A rotary evaporator was used to removewater at a bath temperature of 40° C., to obtain a white solid.

[0094] The molecular weight of the obtained polyepoxysuccinate wasmeasured by means of gel permeation chromatography (GPC). Concretely asample was prepared at a concentration of 200 ppm, and as a standardsubstance, polyethylene glycol with a known molecular weight was used,to draw a calibration curve, for calculating the molecular weight of thesample. The weight average molecular weight of the obtainedpolyepoxysuccinic acid was Mw=20900 (n=100, Mw/Mn=1.00).

EXAMPLES 1 to 3

[0095] Twenty weight percent of sulfuric acid and 0.1 wt % of acorrosion inhibitor shown in Table 1 were added to pure water (electricconductivity 10 μS/cm), to prepare a water-treating microbicide (pH0.6). Stainless steel test pieces (20 mm×30 mm×1 mm) made of SUS316L andpolished with a No. 320 file on the surface were washed with pure waterfor 60 minutes using an ultrasonic washer, washed with acetone for 60minutes, and dried in air. Said water-treating microbicide was dilutedwith seawater (electric conductivity 100 mS/cm) to 100 times(microbicide concentration 1 wt %), to make 100 ml of a testing liquid(pH 1.2), and it was placed in each of ten 100 ml polyethylenecontainers. Said stainless steel test pieces were immersed in thecontainers one by one. The containers were allowed to stand in an 80° C.thermostatic chamber. On the 4^(th) day and 7^(th) day after start ofimmersion, the test pieces were taken out and weighed. The test pieceswere washed with pure water for 5 seconds, washed with acetone for 5seconds, dried in air, and weighed in a silica gel-dried atmosphereusing an electronic balance capable of weighing in 0.01 mg. The averagevalue of five test pieces was obtained. The effect of adding thecorrosion inhibitor was evaluated as described below.

[0096] The weight loss in the period from start of immersion to the4^(th) day (a) and the weight loss in the period from the 4^(th) day to7^(th) day (b) were obtained respectively as follows.

Weight loss (a) (g/m²)=(Weight of test piece before immersion−weight oftest piece on the 4^(th) day)/Surface area of test piece

Weight loss (b) (g/m²)=(Weight of test piece on the 4^(th) day−Weight oftest piece on the 7^(th) day)/Surface area of test piece

[0097] Then, the ratios of the weight losses (a) and (b) caused with theuse of corrosion inhibitor to the weight losses (a) and (b) causedwithout the use of corrosion inhibitor were obtained respectively asdescribed below.

Weight loss (a) ratio=Weight loss (a) caused with use of corrosioninhibitor/Weight loss (a) caused without use of corrosion inhibitor

Weight loss (b) ratio=Weight loss (b) caused with use of corrosioninhibitor/Weight loss (b) caused without use of corrosion inhibitor

[0098] The average value of the weight loss (a) ratio and the weightloss (b) ratio was employed as the weight loss rate. The result is shownin Table 1. (In the table, potassium polyepoxysuccinate is abbreviatedas PES, tetrasodium ethylenediaminetetraacetate, as EDTA, andpolyacrylic acid, as PA.)

Comparative Example 1

[0099] An experiment was carried out as described for Example 1, exceptthat no corrosion inhibitor was added. The result is shown in Table 1.In this comparative example without using any corrosion inhibitor, theweight loss rate was larger than those in Examples 1 to 3, to show thatthe test pieces were corroded heavily. TABLE 1 Corrosion inhibitorWeight loss rate Example 1 PES 0.37 Example 2 EDTA 0.58 Example 3 PA0.67 Comparative Nil 1.00 Example 1

EXAMPLES 4 to 7

[0100] Sulfuric acid was added to seawater (electric conductivity 100mS/cm) for adjusting its pH to 1, and 10 ppm of a corrosion inhibitorshown in Table 2 was added to the seawater, to prepare a testing liquid.Test pieces (20 mm×30 mm×1 mm) made of SUS304 and polished with a No.320 file on the surface were washed with pure water for 60 minutes usingan ultrasonic washer, washed with acetone for 60 minutes, and dried inair. One hundred milliliters of said testing liquid was placed in eachof five 100 ml polyethylene containers, and the stainless steel testpieces were immersed in the containers one by one. The polyethylenecontainers were allowed to stand in a 35° C. thermostatic chamber for 3days, heated to 80° C. and allowed to stand continuously for 17 hours.The test pieces were taken out, washed with pure water for 30 secondsand washed with acetone for 10 seconds. The weight loss due to corrosionwas measured as described below. The weight loss was obtained from thefollowing formula, and the average value of five samples was adopted.

Weight loss (g/m²)=(Weight of test piece before immersion−Weight of testpiece after 3 days of immersion)/Surface area of test piece

[0101] The result is shown in Table 2. (In the table, potassiumpolyepoxysuccinate is abbreviated as PES, tetrasodiumethylenediaminetetraacetate, as EDTA, and butanetetracarboxylic acid, asBTC.)

COMPARATIVE EXAMPLE 2

[0102] An experiment was carried out as described for Example 4, exceptthat no corrosion inhibitor was used.

[0103] As can be seen from Table 2, at a strong acid condition of pH 1,in the case where a corrosion inhibitor was added, a highcorrosion-inhibiting effect was shown compared with the case where nocorrosion inhibitor was added. TABLE 2 Concentration Weight lossCorrosion inhibitor (ppm) (g/m²) Example 4 PES 10 0.19 Example 5 EDTA 100.14 Example 6 PA 10 0.38 Example 7 BTC 10 17.9 Comparative Nil — 27.9Example 2

EXAMPLE 8 AND COMPARATIVE EXAMPLES 3 AND 4

[0104] Twenty weight percent of sulfuric acid and 0.1 wt % of acorrosion inhibitor shown in Table 3 were added to pure water (electricconductivity 10 μS/cm), to prepare a microbicide for a water treatmentapparatus. In Example 8, 0.5 wt % each of sodium citrate and malic acidwere further added as storage stabilizers. Stainless steel test pieces(20 mm×30 mm×1 mm) made of SUS304 and polished with a No. 320 file onthe surfaces were washed with pure water for 60 minutes using anultrasonic washer, washed with acetone for 60 minutes, and dried in air.Then, the stainless steel test pieces were immersed in 50° C. 20% nitricacid water for passivation treatment for 1 hour, taken out, washed withacetone and dried in air. Said microbicide was diluted with seawater(100 mS/cm) to 100 times (microbicide concentration 1 wt %), to make atesting liquid (pH 1.4), and the testing liquid was placed in five 100ml polyethylene containers. Said stainless steel test pieces wereimmersed in the containers one by one. The polyethylene containers wereallowed to stand in an 80° C. thermostatic chamber. On the 6^(th) dayafter start of immersion, the test pieces were taken out and weighed.The test pieces were washed with pure water for 5 seconds, washed withacetone for 5 seconds, dried in air, and weighed in a silica gel-driedatmosphere using an electronic balance capable of weighing in 0.01 mg.The weight loss was calculated as described for Example 4. The result isshown in Table 3.

[0105] Separately seawater with a salt concentration of 6.9 wt % wasallowed to stand at 30° C. overnight to stabilize the plate count, anddiluted with sterile water to 3.5 wt % (this is called solution A). Toseawater (electric conductivity 100 mS/cm), 0.1 wt % of a microbicideshown in Table 3 was added (pH 3.1) and the solution was allowed tostand at 30° C. for 30 minutes (this is called solution B). Forobtaining the plate count, a medium for marine bacteria was used toculture at 30° C. for 6 days, and the number of emerging colonies wascounted. The plate count remaining rate to the plate count obtainedwithout pH regulation (solution A) was obtained. That is, the platecount remaining rate was obtained from the following formula.

Plate count remaining rate (%)=[{Plate count after reaction (solutionB)}/{Plate count without pH regulation (solution A)}]×100

[0106] The result is shown in Table 3.

[0107] The microbicide was separately allowed to stand in a 25° C.thermostatic chamber, and the solution state was confirmed on the19^(th) day. The result is shown in Table 3. (In the table, polyacrylicacid is abbreviated as PA, sodium citrate, as CA, and malic acid, asMA.)

[0108] As can be seen from Table 3, compared with Comparative Example 3in which no storage stabilizer was added, Example 8 in which a storagestabilizer was added showed higher storage stability in addition to thecorrosion-inhibiting effect. Comparative Example 4 in which no corrosioninhibitor was added was large in the weight loss of test pieces. TABLE 3Plate Weight count Corrosion Storage loss remaining Solution inhibitorstabilizer (g/m²) rate (%) state Example 8 PA CA, MA 0.19 0.12 NoPrecipitation Comparative PA Nil 0.24 0.08 Rather Example 3 cloudyComparative Nil Nil 1.08 0.16 No Example 4 precipitation

[0109] Industrial Applicability:

[0110] The present invention can achieve effective sterilization whileinhibiting the corrosion of equipment piping in water treatment using amembrane separation device. Therefore, sterilization frequency can beincreased, and pH can be lowered further, to increase the sterilizationeffect.

[0111] Furthermore, in the case where a storage stabilizer is added tothe water-treating microbicide of this invention, high storage stabilitycan be realized while the sterilization effect and thecorrosion-preventing effect are sustained.

[0112] The present invention can be especially suitably used for theprocesses of seawater desalination, brackish water desalination, etc.

1. A water-treating microbicide, comprising an inorganic acid, acorrosion inhibitor and a carboxylic acid having 8 or less carbon atomsor any of alkali metal salts thereof.
 2. A water-treating microbicide,according to claim 1, wherein the corrosion inhibitor is polyacrylicacid.
 3. A water-treating microbicide, according to claim 1, wherein theinorganic acid is sulfuric acid.
 4. A water-treating microbicide,according to claim 1, wherein the concentration of the inorganic acid isin a range of 50 ppm (weight) to 50 wt %, and the concentration of thecorrosion inhibitor is in a range of 50 ppm (weight) to 50 wt %.
 5. Awater-treating microbicide, according to claim 2, wherein the molecularweight of polyacrylic acid is 500 to 10,000.
 6. A water-treatingmicrobicide, according to claim 1, wherein the carboxylic acid having 8or less carbon atoms is at least one selected from acetic acid, tartaricacid, succinic acid, citric acid and malic acid.
 7. A water-treatingmicrobicide, according to claim 1, which is used in a water treatmentprocess using a separation membrane.
 8. A water-treating microbicide,according to claim 7, wherein a reverse osmosis membrane is used as theseparation membrane.
 9. A water-treating microbicide, according to claim1, which is used in a water treatment process for producing drinkingwater.
 10. A water treatment method, comprising the step of adding thewater-treating microbicide as set forth in claim 1 to a liquidundergoing treatment in any step before a membrane separation step in awater treatment process using a separation membrane.
 11. A watertreatment method, according to claim 10, wherein the water-treatingmicrobicide is added in a range of 10 ppm (weight) to 10 wt %.
 12. Awater treatment method, according to claim 10, wherein thewater-treating microbicide is added intermittently.
 13. A watertreatment method, according to claim 12, wherein the water-treatingmicrobicide is kept added for 0.5 to 2.5 hours each time.
 14. A watertreatment method, according to claim 12, wherein the water-treatingmicrobicide is added at a frequency of once per day to per month.
 15. Awater treatment method, according to claim 10, wherein thewater-treating microbicide is added during membrane separation.
 16. Awater treatment method, according to claim 10, wherein a reverse osmosismembrane is used as the separation membrane.
 17. A water treatmentmethod, according to claim 10, which is used for producing drinkingwater.
 18. A water treatment method, according to claim 10, whereinseawater is used as the liquid undergoing treatment.
 19. A watertreatment method, comprising the steps of adding an inorganic acid to aliquid undergoing treatment for intermittently keeping the liquid at pH4 or less and adding a corrosion inhibitor to the liquid undergoingtreatment, in any steps before a membrane separation step in a watertreatment process using a separation membrane.
 20. A water treatmentmethod, according to claim 19, wherein the inorganic acid is kept addedfor 0.5 to 2.5 hours each time.
 21. A water treatment method, accordingto claim 19, wherein the inorganic acid is added at a frequency of onceper day to per month.
 22. A water treatment method, according to claim19, wherein in the steps for intermittently keeping the liquid at pH 4or less, the pH is kept at 3 or less at a frequency of once per 2 to1,000 times, and the pH is kept at larger than 3 in the other cases. 23.A water treatment method, according to claim 19, wherein the inorganicacid is sulfuric acid.
 24. A water treatment method, according to claim19, wherein the corrosion inhibitor is polyacrylic acid.
 25. Awater-treating microbicide, according to claim 24, wherein the molecularweight of polyacrylic acid is 500 to 10,000.
 26. A water treatmentmethod, according to claim 19, wherein the inorganic acid is added in arange of 10 ppm (weight) to 1 wt % and the corrosion inhibitor is addedin a range of 0.1 ppm (weight) to 1 wt %.
 27. A water treatment method,according to claim 19, wherein the inorganic acid is added on thedownstream side of adding the corrosion inhibitor to the liquidundergoing treatment.
 28. A water treatment method, according to claim22, wherein the corrosion inhibitor is added when the pH of the liquidundergoing treatment is 3 or less.
 29. A water treatment method,according to claim 19, wherein the separation membrane is a reverseosmosis membrane.
 30. A water treatment method, according to claim 19,which is used for producing drinking water.
 31. A water treatmentmethod, according to claim 19, wherein seawater is used as the liquidundergoing treatment.
 32. A water treatment method, comprising the stepsof intermittently adding an inorganic acid and a corrosion inhibitor toa liquid undergoing treatment to achieve a plate count remaining rate of30% or less in any steps before a membrane separation step in a watertreatment process using a separation membrane and to achieve a platecount remaining rate of 15% or less at a frequency of once per 2 to1,000 times in said steps of intermittently adding the inorganic acid.33. A water treatment apparatus having a membrane separation device,comprising a means for adding an aqueous solution containing aninorganic acid and a corrosion inhibitor to a liquid undergoingtreatment to be fed to said membrane separation device.
 34. A watertreatment apparatus having a membrane separation device, comprising ameans for feeding an aqueous solution containing an acid and a means forfeeding an aqueous solution containing a corrosion inhibitor to a liquidundergoing treatment to be fed to said membrane separation device.
 35. Awater-treating microbicide, according to claim 33 or 34, wherein thecorrosion inhibitor is polyacrylic acid.
 36. A water treatmentapparatus, according to claim 33 or 34, wherein the acid is sulfuricacid.
 37. A water treatment apparatus, according to claim 33, whereinsaid aqueous solution contains further a carboxylic acid having 8 orless carbon atoms or any of alkali metal salts thereof.
 38. A watertreatment apparatus, according to claim 33 or 34, wherein the means forfeeding said aqueous solution(s) is/are a mean/means for intermittentlyfeeding the aqueous solution(s).
 39. A water treatment apparatus,according to claim 33 or 34, wherein the separation membrane is areverse osmosis membrane.
 40. A water treatment apparatus, according toclaim 33 or 34, which is used for producing drinking water.
 41. A watertreatment apparatus, according to claim 33 or 34, wherein the liquidundergoing treatment is seawater.